Following the Battle of Paoli in Pennsylvania in 1777 he became known as "No-flint Grey" for reputedly ordering his men to extract the flints from their muskets during a night approach and to fight with bayonets only.

He later fought in the French Revolutionary Wars (1792–1802), capturing Martinique as leverage to force the Treaty of Amiens, and was appointed as Governor of Guernsey.

Because he had two older brothers, Grey did not expect to inherit his father's titles and estates, so he pursued a career in the army.

In the Seven Years' War, he served as adjutant in the staff of Duke Ferdinand of Brunswick and on 1 August 1759 was wounded at Minden.

He earned the nickname "No-flint Grey" after the Battle of Paoli in the same campaign when, to ensure surprise in the night attack on an American encampment, it was said he ordered the infantry of his command to remove the flints from their muskets and use only their bayonets.

Immediately following his disastrous retreat at the Battle of Monmouth, the American General Charles Lee excused himself from criticism by complaining that he had directly faced the advance Grey's 3rd brigade, suggesting the Earl was a feared and respected opponent by this stage in the war.

On 27 September 1778, Grey used the same methods as he had at the Battle of Paoli[citation needed] in a night attack at Old Tappan, New Jersey, which came to be known as the Baylor Massacre.

At the outset of the French Revolutionary Wars in 1793, Sir Charles Grey was appointed commander of the West Indian expedition.

Their children were: Grey and his wife brought up Eliza Courtney, the illegitimate daughter of their son Charles with Georgiana Cavendish, Duchess of Devonshire.